1. Field of Invention
The invention relates to medical diagnosis and, in particular to a method of measuring the instantaneous arterial blood pressure in a non-invasive approach and the device thereof.
2. Related Art
Cardiovascular diseases have become one of the major death causes of human beings. Therefore, the study of blood vessel properties is one of the major medical research fields. The emphases have been on such behaviors as blood pressure, blood flow, and elasticity. The measurement of the instantaneous blood pressure of blood vessels is thus extremely important.
In 1876, Marey first invented the Sphygmograph to take the blood pressure. Afterwards, more researchers have devoted themselves in non-invasive, convenient and reliable equipment in order to take the arterial blood pressure in medical diagnosis and health care. Clinically, the blood pressure is almost measured using non-invasive intermittent techniques. The most common technique is to use auscultatory and computerized oscillometric method.
Later on, three non-invasive approaches have been proposed to measure the instantaneous intra-arterial blood pressure. The first is the employ the arterial tonometry based on coplanar measurement (Pressman et al. 1963 and Mackay et al. 1964). The second is the Peñáz's method (Peñáz's 1973 and Wesseling 1984, 1986). The third one is the volume-compensation method (Yamakoshi et al. 1979, 1980). The latter two are based upon the vascular unloading technique (Geddes 1970, Shirer 1962 and O'brien et al. 1991).
Current artery tonometrys include, for example, HDI CvProfilor (Cohn et al. 1995), Sphygmocor (Giot et al. 1996), Pulsepen (Salv et al. 2004), and Colin radial tonometry, all using the principle of arterial tonometry. In these cases, some artery of one arm is urged by a hard structure and partially compressed. Therefore, the pulsation waveform can be continuously read out without using an occluding cuff. However, to calibrate the obtained blood pressure measurement, one needs to use the auscultatory or cuff-oscillometric method to measure the systolic blood pressure (SBP), diastolic blood pressure (DBP), and mean blood pressure (MBP) of the other arm. The intra-arterial blood pressure is then estimated in proportion. Nonetheless, the transmission property of blood pressure from the artery to the skin above it is nonlinear.
The above-mentioned vascular unloading technique involves the Finapres type and the Portapres type, both performing continuous monitoring of the arterial blood pressure at fingers in a non-invasive approach. With the volume-clamp method disclosed by Peñáz, the finger arteries are clamped at a fixed diameter by applying an external pulsating pressure via an inflatable bladder mounted in a finger cuff and a fast-acting servo system. Finapres uses the Wesseling standard to determine a setpoint. The diameter of where the finger artery is clamped is determined by an infrared (IR) plethysmograph. The plethysmograph is disposed inside the finger cuff so that the transmural pressure is zero. At the same time, the cuff pressure is equal to the intra-arterial blood pressure. This equality assumes that the pulsating pressure has a diminishing loss from the artery to the cuff. Afterwards, Yamakoshi et. al. propose a volume-compensation method to improve the servo-reference and develop a local pressurization technique. These lead to a pad-type cuff sphygmomanometer for finger and wrist (Tanaka et al. 2005). Such a sphygmomanometer can prevent discomfort on the biological segment that is surrounded by the occluding cuff during a long-term measurement.
However, all the above-mentioned methods cannot obtain the true instantaneous blood pressure of the artery vessel. This is because these methods do not take into account the pressure transmission of the muscle tissues. Experience tells us that these conventional techniques cannot render accurate behaviors of arterial vessels.
The key issues are how to obtain the true instantaneous intra-arterial blood pressure and how to obtain characteristics of blood vessels such as the variation in their diameters. That is, it is desirable to obtain the simple variation of blood vessels. An objective of the invention is to employ a controlled method so that the muscle tissues are compliantly invariant, thereby rendering the simple behavior of blood vessel variations. At the same time, the characteristics of blood vessels are identified in order to estimate the arterial instantaneous blood pressure and its dynamic arterial dynamic compliance at the critical position.